This invention relates to a variable flow turbocharger, and in particular the turbine stage of a turbocharger for an internal combustion engine.
Many internal combustion engines are equipped with turbochargers to improve engine efficiency. A turbocharger comprises a turbine and a compressor. In operation, the turbine captures high-temperature exhaust gas coming from the engine exhaust manifold. This exhaust gas then is used to drive the compressor which, in turn, pumps high pressure air into the engine inlet and combustion chambers. The effect of this process in an internal combustion engine is to increase the volume of air available for combustion. Because more air is available, a correspondingly greater amount of fuel can be consumed, or burnt, per cycle. In theory, the greater the fuel burnt, the greater the horsepower.
Typically, the turbine stage of a turbocharger comprises a turbine chamber within which the turbine is mounted, an inlet passageway arranged around the turbine chamber for introducing exhaust gas into the turbine chamber, and an outlet passageway extending from the turbine chamber for discharging the exhaust gas. The turbine chamber and the inlet and outlet passageways communicate such that incoming exhaust gas flows through the inlet passageway to the outlet passageway via the turbine chamber and rotates the turbine.
Under the current state of the art, it is known to vary the flow of exhaust gas in the turbine stage so that the power output of the turbine can be adjusted to suit varying engine demands. One common type of variable flow turbocharger is a wastegated (turbine bypass) turbocharger. Such turbochargers have a wastegate for bypassing exhaust gas around the turbine using a valve in the inlet passageway controlled by actuator means. Wastegated turbochargers are usually matched to give good performance at low engine speed with the valve closed. This improves transient response and reduces exhaust temperatures and emissions. As engine speed increases, the wastegate valve begins to open. This has the effect of increasing the flow capacity of the turbine stage and avoiding excess air delivery and turbine overspeed (see U.S. Pat. No. 4,643,640 for the basic concept of wastegated turbochargers).
In another type of variable flow turbocharger, a more complex method of turbocharging uses a turbine stage where the flow capacity of the turbine stage is adjusted by varying the geometry of a nozzle, the part of the inlet passageway which surrounds the turbine and directs the exhaust gas at the turbine. One common type of variable nozzle turbocharger has a set of swing or slide vanes which extend into the nozzle and which can be caused to vary in orientation so as to increase or decrease the effective cross-sectional area between the vanes. Decreasing the effective cross-sectional area between the vanes permits turbine speed to be increased by increasing the pressure differential across the turbine (see U.S. Pat. Nos. 4,643,640, 4,654,941 and 4,659,295 for the basic concept of swing vanes). In another type of variable nozzle turbocharger, one wall of the nozzle is defined by a moveable wall member, generally referred to as a nozzle ring. The position of the ring relative to a facing wall of the nozzle is adjustable to control the width of the nozzle. For instance, as gas flowing through the turbine decreases, the nozzle width may also be decreased to maintain gas velocity and optimize turbine output (see EP 1 226 580 A2 for the basic concept of a nozzle ring).